785 MPa LEVEL EXTRA-THICK QUENCHED AND TEMPERED RACK STEEL PLATE FOR OFFSHORE PLATFORMS AND PREPARATION METHOD THEREFOR

ABSTRACT

The present invention discloses a 785 MPa level extra-thick quenched and tempered rack steel plate for offshore platforms and a preparation method therefor, wherein chemical components of the steel plate by weight percentage are: 0.08%-0.15% of C, 0.60%-1.5% of Mn, 0.10%-0.50% of Si, 0.002%-0.005% of S, 0.005%-0.015% of P, 2.7%-4.0% of Ni, 0.40%-1.00% of Cr, 0.30%-0.70% of Mo, 0.10%-0.40% of Cu, 0%-0.080% of Nb, 0%-0.10% of V, 0%-0.08% of Ti, 0.02%-0.08% of Al, 0.0008%-0.0015% of B, and the balance of Fe and other unavoidable impurities. The present invention adopts a low C, high Ni—Cr—Mo alloying and Nb—V—Ti microalloying component system, has a yield strength of ≥795 MPa, a tensile strength of ≥865 MPa, a −60° C. Charpy V-notch impact energy of ≥140 J, a Z value of ≥35%, and a maximum thickness of 100 mm, and can be used to manufacture racks for offshore platforms.

TECHNICAL FIELD

The present invention belongs to the field of thick plate and extra-thick plate production, and particularly relates to a 785 MPa level high-hardenability high-strength high-toughness extra-thick quenched and tempered rack steel plate for offshore platforms and a preparation method therefor.

BACKGROUND

Increasingly prominent energy crisis promotes the overexploitation of terrestrial high-quality oil resources, and at the same time accelerates the exploration and exploitation of offshore oil and gas by countries around the world. Gradual extension of offshore oil and gas exploitation from land to offshore and deep seas has become a development trend in energy field of the world. Oil and gas resources in sea areas of our country are very rich, especially in the South China Sea. In order to exploit and utilize marine resources, our country has continuously developed and upgraded the technical level of marine equipment. At present, ship and marine engineering steels in our country can meet most of the needs in domestic market, but some high-level special steels still rely on imports, mainly including steel plates with high strength, lamellar tearing resistance, ultra-low-temperature toughness, etc., which have very strict production process, high requirements for equipment, and difficulty in development. The most typical steel used in marine engineering is an extra-thick rack steel plate for jack-up drilling platform legs. This steel plate is used in severe marine environments such as waves, ocean tides and ice for a long time, and bears severe loads and complex corrosive environments. Therefore, an extra-thick rack steel is required to have the performances such as high strength, high toughness, fatigue resistance, corrosion resistance, low-temperature toughness, lamellar tearing resistance and good weldability.

In order to improve the safety of steels for offshore platforms, the use of high-strength, high-toughness and large-thickness steel plates is increasing year by year. At present, the research and development of the extra-thick rack steel plate has made important progress. The thickness of a domestic 690 MPa level steel plate has reached 215 mm. For the reasons that the core of a steel ingot or billet has the defects such as looseness, shrinkage cavities and segregation, rolling deformation is difficult to penetrate into the core, and quenching cooling rate and uniformity are poor, the structural performances in the thickness direction of a current 690 MPa level rack steel for offshore platforms are seriously nonuniform, and especially, the core has low strength and poor toughness. As the scales of offshore platforms continue to increase, the weight of higher strength level rack steels is required to be reduced to ensure the safety of the offshore platforms. Therefore, it is necessary to develop a high-hardenability, high-toughness, and high-strength, such as 785 MPa level rack steel for offshore platforms to replace the existing 690 MPa level rack steel.

With the continuous progress of offshore equipment and technology, the problem to be urgently solved is to use a more reasonable high-hardenability alloy design and a production process thereof to make an extra-thick rack steel plate for offshore platforms have higher strength, better low-temperature toughness and good structural performance uniformity in the thickness direction, which will also provide a guarantee for the development of offshore oil and gas to extremely-low-temperature sea areas and deeper sea areas.

SUMMARY

The purpose of the present invention are to provide a 785 MPa level high-hardenability high-strength high-toughness extra-thick quenched and tempered rack steel plate for offshore platforms and a manufacturing method therefor, which adopts a low C, high Ni—Cr—Mo alloying and Nb-V microalloying component system. The extra-thick quenched and tempered steel plate has a structure of tempered martensite, has a yield strength of ≥795 MPa, a tensile strength of ≥865 MPa, a −60° C. Charpy V-notch impact energy of ≥140 J, a Z value of ≥35%, and a maximum thickness of 100 mm, and can be used to manufacture racks for offshore platforms.

To achieve the above purpose, the present invention adopts the following technical solution:

A 785 MPa level extra-thick quenched and tempered rack steel plate for offshore platforms, which adopts a low C, high Ni—Cr—Mo alloying and Nb—V—Ti microalloying design, wherein chemical components of the steel plate by weight percentage are: 0.08%-0.15% of C, 0.60%-1.5% of Mn, 0.10%-0.50% of Si, 0.002%-0.005% of S, 0.005%-0.015% of P, 2.7%-4.0% of Ni, 0.40%-1.00% of Cr, 0.30%-0.70% of Mo, 0.10%-0.40% of Cu, 0%-0.080% of Nb, 0%-0.10% of V, 0%-0.08% of Ti, 0.02%-0.08% of Al, 0.0008%-0.0015% of B, and the balance of Fe and other unavoidable impurities.

The metallographic structure of the 785 MPa level extra-thick quenched and tempered rack steel plate for offshore platforms of the present invention is a tempered martensite structure.

Further, the 785 MPa level extra-thick quenched and tempered rack steel plate for offshore platforms of the present invention has a yield strength of 795-980 MPa, a tensile strength of 865-980 MPa, an elongation of 16%-19%, and a section shrinkage percentage of 56%-63%.

Further, the 785 MPa level extra-thick quenched and tempered rack steel plate for offshore platforms of the present invention has a −60° C. impact energy of ≥140 J on a test sample with a dimension of 10 mm×10 mm×55 mm.

Preferably, the 785 MPa level extra-thick quenched and tempered rack steel plate for offshore platforms has a thickness of 80-100 mm.

Another purpose of the present invention is to provide a preparation method for the 785 MPa level extra-thick quenched and tempered rack steel plate for offshore platforms.

The preparation method for the 785 MPa level extra-thick quenched and tempered rack steel plate for offshore platforms, comprising the following process steps:

{circle around (1)} Forging a casting blank with a thickness of 180-210 mm into a billet with a thickness of 160-170 mm, heating the billet together with a furnace to 1100° C.-1200° C. and soaking for 2-4 hours, wherein chemical components of the billet by weight percentage are: 0.08%-0.15% of C, 0.60%-1.5% of Mn, 0.10%-0.50% of Si, 0.002%-0.005% of S, 0.005%-0.015% of P, 2.7%-4.0% of Ni, 0.40%-1.00% of Cr, 0.30%-0.70% of Mo, 0.10%-0.40% of Cu, 0%-0.080% of Nb, 0%-0.10% of V, 0%-0.08% of Ti, 0.02%-0.08% of Al, 0.0008%-0.0015% of B, and the balance of Fe and other unavoidable impurities;

{circle around (2)} Hot rolling the billet into a hot-rolled steel plate with a thickness of 80-100 mm through a rough rolling stage and a finish rolling stage, wherein the rough rolling stage has an initial rolling temperature of 1050° C.-1150° C., and a final rolling temperature of 980° C.-1020° C.; the finish rolling stage has an initial rolling temperature of 850° C.-870° C., and a final rolling temperature of 840° C.-860° C.; and the steel plate is air cooled to room temperature after rolling.

{circle around (3)} Placing the hot-rolled steel plate cooled to room temperature into a heating furnace for thermal refining (quenching+tempering), and air cooling to room temperature after tempering to obtain a product;

Preferably, in step {circle around (2)}, the number of rough rolling passes is 3-5, and the reduction in each pass is 9%-14%; the number of finish rolling passes is 2-4, and the reduction in each pass is 7%-12%.

Preferably, in step {circle around (3)}, the thermal refining process is that: quenching temperature is 900° C.-930° C., soaking time is 90-120 minutes, and quenching water temperature is 20° C.; tempering temperature is 600° C.-660° C., and tempering soaking time is 60-90 minutes.

The present invention has the following beneficial effects:

In order to ensure that the extra-thick plate has good hardenability, low-temperature toughness and structural uniformity, the present invention notably increases the hardenability of the steel plate in a low C, high Ni—Cr—Mo alloying and Nb—V—Ti microalloying component design manner, and a complete martensite structure can be obtained under the condition that cooling rate is ≥0.5° C./s. The present invention adopts a low-carbon component design, the content of C by weight percentage is 0.08%-0.15%, and the low carbon content reduces the welding carbon equivalent and improves the weldability of the steel plate. Nanometer level precipitation phases obtained by Nb—V—Ti microalloying treatment can have a strong precipitation strengthening effect and improve the strength of the steel plate. Through a high Ni component design, the low-temperature impact toughness of the steel plate is improved. A good match of the strength and toughness of the steel plate is achieved by controlling a quenching and tempering heat treatment process.

The 785 MPa level extra-thick quenched and tempered rack steel plate for offshore platforms prepared by the present invention has the characteristics of high toughness, high structural performance uniformity, etc., and the ¼ thickness and ½ thickness positions of the steel plate have a yield strength of 795-980 MPa, a tensile strength of 865-980 MPa, an elongation of 16%-19%, a section shrinkage percentage of 56%-63%, and a −60° C. impact energy of ≥140 J. In addition, the 785 MPa level high-hardenability high-strength high-toughness extra-thick quenched and tempered rack steel plate for offshore platforms of the present invention also has a characteristic of low yield ratio, the application range is wider, the structural safety is higher, and the steel plate is suitable for a working environment in an extremely low temperature sea area.

DESCRIPTION OF DRAWINGS

FIG. 1 is a process schematic diagram of a preparation method of the present invention.

FIG. 2 is a metallographic structure of a ½ thickness position of a quenched and tempered steel plate prepared by embodiment 1;

FIG. 3 is a metallographic structure of a ½ thickness position of a quenched and tempered steel plate prepared by embodiment 2; and

FIG. 4 is a TEM morphology image of a ½ thickness position of a quenched and tempered steel plate prepared by embodiment 3.

DETAILED DESCRIPTION

The following non-limiting embodiments may enable those ordinary skilled in the art to fully understand the present invention, but do not limit the present invention in any way.

The test methods described in the following embodiments are conventional methods unless otherwise specified; and the reagents and materials are commercially available unless otherwise specified.

The hot rolling mill used in the following embodiments is a Φ450 mm hot rolling mill; The heating furnace used during the heat treatment in the following embodiments is a high-temperature box-type resistance furnace, and the specification model is NEUXXSL1200-40;

The transmission electron microscope used in the following embodiments is FEI Tecnai G2 F20 field emission transmission electron microscope; and

The optical microscope used in the following embodiments is an OLYMPUS BX53M multifunctional optical microscope.

Embodiment 1

An extra-thick rack steel plate for offshore platforms with a thickness of 100 mm is prepared by the following process steps:

Heating a forged billet with a thickness of 170 mm together with a furnace to 1200° C. and soaking for 3 hours, wherein chemical components of the billet by weight percentage are: 0.13% of C, 1.2% of Mn, 0.27% of Si, 0.0021% of S, 0.005% of P, 3.7% of Ni, 0.66% of Cr, 0.6% of Mo, 0.20% of Cu, 0.04% of V, 0.048% of Al, 0.0010% of B, and the balance of Fe and other unavoidable impurities; and hot rolling the billet into a hot-rolled steel plate with a thickness of 100 mm by 5 passes. The initial rolling temperature and final rolling temperature of the rough rolling stage are respectively 1100° C. and 1020° C., the billet is rough rolled to 120 mm by 3 passes, and the reduction in each pass is 10%-12%; the initial rolling temperature and final rolling temperature of the finish rolling stage are respectively 850° C. and 845° C., the billet is finish rolled to 100 mm by 2 passes, and the reduction in each pass is 7%-8%. Air cooling the steel plate to room temperature after the hot rolling. Then performing off-line thermal refining: placing the hot-rolled steel plate cooled to room temperature into a high-temperature box-type resistance furnace with a temperature of 910° C. for quenching, soaking for 120 minutes and water quenching; placing the quenched steel plate into a high-temperature box-type resistance furnace with a temperature of 660° C. for tempering, soaking for 90 minutes, removing from the furnace and air cooling to room temperature, as shown in FIG. 1.

The structure of the steel plate is tempered martensite, as shown in the OM morphology of FIG. 2. As tested, the steel plate has a yield strength of 795 MPa, a tensile strength of 865 MPa, an elongation of 16%, a section shrinkage percentage of 61%, and a −60° C. impact energy of 202 J on a test sample with a thickness of 10 mm.

Embodiment 2

An extra-thick rack steel plate for offshore platforms with a thickness of 90 mm is prepared by the following process steps:

Heating a forged billet with a thickness of 162 mm together with a furnace to 1200° C. and soaking for 3 hours, wherein chemical components of the billet by weight percentage are: 0.13% of C, 1.1% of Mn, 0.26% of Si, 0.0026% of S, 0.005% of P, 3.0% of Ni, 0.65% of Cr, 0.6% of Mo, 0.20% of Cu, 0.04% of V, 0.024% of Al, 0.0010% of B, and the balance of Fe and other unavoidable impurities; and hot rolling the billet into a hot-rolled steel plate with a thickness of 90 mm by 6 passes. The initial rolling temperature and final rolling temperature of the rough rolling stage are respectively 1065° C. and 1000° C., the billet is rough rolled to 115 mm by 3 passes, and the reduction in each pass is 10%-12%; the initial rolling temperature and final rolling temperature of the finish rolling stage are respectively 850° C. and 848° C., the billet is finish rolled to 90 mm by 3 passes, and the reduction in each pass is 7%-9%. Air cooling the steel plate to room temperature after the hot rolling. Then performing off-line thermal refining: placing the hot-rolled steel plate cooled to room temperature into a high-temperature box-type resistance furnace with a temperature of 910° C. for quenching, soaking for 120 minutes and water quenching; placing the quenched steel plate into a high-temperature box-type resistance furnace with a temperature of 600° C. for tempering, soaking for 90 minutes, removing from the furnace and air cooling to room temperature, as shown in FIG. 1.

The structure of the steel plate is tempered martensite, as shown in the OM morphology of FIG. 3. As tested, the steel plate has a yield strength of 900 MPa, a tensile strength of 980 MPa, an elongation of 19%, a section shrinkage percentage of 56%, and a −60° C. impact energy of 140 J on a test sample with a thickness of 10 mm.

Embodiment 3

An extra-thick rack steel plate for offshore platforms with a thickness of 80 mm is prepared by the following process steps:

Heating a forged billet with a thickness of 160 mm together with a furnace to 1200° C. and soaking for 3 hours, wherein chemical components of the billet by weight percentage are: 0.123% of C, 1.1% of Mn, 0.26% of Si, 0.0015% of S, 0.004% of P, 2.9% of Ni, 0.60% of Cr, 0.5% of Mo, 0.21% of Cu, 0.03% of Nb, 0.04% of V, 0.010% of Ti, 0.024% of Al, 0.0010% of B, and the balance of Fe and other unavoidable impurities; and hot rolling the billet into a hot-rolled steel plate with a thickness of 90 mm by 6 passes. The initial rolling temperature and final rolling temperature of the rough rolling stage are respectively 1085° C. and 1010° C., the billet is rough rolled to 110 mm by 3 passes, and the reduction in each pass is 11%-13%; the initial rolling temperature and final rolling temperature of the finish rolling stage are respectively 858° C. and 850° C., the billet is finish rolled to 80 mm by 3 passes, and the reduction in each pass is 9%-11%. Air cooling the steel plate to room temperature after the hot rolling. Then performing off-line thermal refining: placing the hot-rolled steel plate cooled to room temperature into a high-temperature box-type resistance furnace with a temperature of 930° C. for quenching, soaking for 90 minutes and water quenching; placing the quenched steel plate into a high-temperature box-type resistance furnace with a temperature of 650° C. for tempering, soaking for 60 minutes, removing from the furnace and air cooling to room temperature, as shown in FIG. 1.

The structure of the steel plate is tempered martensite, as shown in the TEM morphology of FIG. 4. As tested, the steel plate has a yield strength of 805 MPa, a tensile strength of 890 MPa, an elongation of 19%, a section shrinkage percentage of 63%, and a −60° C. impact energy of 200 J on a test sample with a thickness of 10 mm.

Besides the above embodiments, the present invention can also have other embodiments. Technical solutions formed by adopting equivalent replacement or equivalent transformation shall be included in the protection scope claimed in the present invention. 

1. A 785 MPa level extra-thick quenched and tempered rack steel plate for offshore platforms, wherein chemical components of the extra-thick quenched and tempered steel plate by weight percentage are: 0.08%-0.15% of C, 0.60%-1.5% of Mn, 0.10%-0.50% of Si, 0.002%-0.005% of S, 0.005%-0.015% of P, 2.7%-4.% of Ni, 0.40%-1.00% of Cr, 0.30%-0.70% of Mo, 0.10%-0.40% of Cu, 0%-0.080% of Nb, 0%-0.10% of V, 0%-0.08% of Ti, 0.02%-0.08% of Al, 0.0008%-0.0015% of B, and the balance of Fe and other unavoidable impurities.
 2. The 785 MPa level extra-thick quenched and tempered rack steel plate for offshore platforms according to claim 1, wherein the extra-thick quenched and tempered steel plate has a thickness of 80-100 mm.
 3. The 785 MPa level extra-thick quenched and tempered rack steel plate for offshore platforms according to claim 1, wherein the structure of the extra-thick quenched and tempered steel plate is a tempered martensite structure.
 4. The 785 MPa level extra-thick quenched and tempered rack steel plate for offshore platforms according to claim 1, wherein the extra-thick quenched and tempered steel plate has a yield strength of 795-980 MPa, a tensile strength of 865-980 MPa, an elongation of 16%-19%, and a section shrinkage percentage of 56%-63%.
 5. The 785 MPa level extra-thick quenched and tempered rack steel plate for offshore platforms according to claim 1, wherein the extra-thick quenched and tempered steel plate has a −60° C. impact energy of ≥140 J.
 6. Preparation method for the 785 MPa level extra-thick quenched and tempered rack steel plate for offshore platforms according to claim 1, comprising the following process steps: {circle around (1)} forging a casting blank with a thickness of 180-210 mm into a billet with a thickness of 160-170 mm, heating the billet together with a furnace to 1100° C.-1200° C. and soaking for 2-4 hours, wherein chemical components of the billet by weight percentage are: 0.08%-0.15% of C, 0.60%-1.5% of Mn, 0.10%-0.50% of Si, 0.002%-0.005% of S, 0.005%-0.015% of P, 2.7%-4.0% of Ni, 0.40%-1.00% of Cr, 0.30%-0.70% of Mo, 0.10%-0.40% of Cu, 0%-0.080% of Nb, 0%-0.10% of V, 0%-0.08% of Ti, 0.02%-0.08% of Al, 0.0008%-0.0015% of B, and the balance of Fe and other unavoidable impurities; {circle around (2)} rolling the billet into a hot-rolled steel plate with a thickness of 80-100 mm through a rough rolling stage and a finish rolling stage, wherein the rough rolling stage has an initial rolling temperature of 1050° C.-1150° C., and a final rolling temperature of 980° C.-1020° C.; the finish rolling stage has an initial rolling temperature of 850° C.-870° C., and a final rolling temperature of 840° C.-860° C.; and the steel plate is air cooled to room temperature after rolling; {circle around (3)} placing the hot-rolled steel plate cooled to room temperature into a heating furnace for thermal refining including quenching and tempering, and air cooling to room temperature after tempering to obtain a product.
 7. The method according to claim 6, wherein in step {circle around (2)}, the number of rough rolling passes is 3-5, and the reduction in each pass is 9%-14%; the number of finish rolling passes is 2-4, and the reduction in each pass is 7%-12%.
 8. The method according to claim 6, wherein in step {circle around (3)}, the thermal refining process is that: quenching temperature is 900° C.-930° C., soaking time is 90-120 minutes, and quenching water temperature is 20° C.; tempering temperature is 600° C.-660° C., and tempering soaking time is 60-90 minutes. 